In order to detect and image gas clouds, especially through the use of infrared detection systems over a wide spectral range, it is often necessary to spectrally limit the incoming radiation to selected wavelength bands using spectral filtering techniques. This is accomplished by measuring the radiation emitted by a background of the gas cloud in two different wavelength bands, one which is absorbed by the gas cloud, and one which is not absorbed by the gas cloud.
Devices which can detect and image gas clouds have a wide range of applications, such as, for example, constant monitoring of a scene in industrial installations and facilities for gas leakages, and identifying escaping gases from gas transporting vehicles subsequent to traffic accidents. Typically, detectors which detect radiation in the visible spectral range are of lower cost than infrared detectors. However, since most hazardous gases of interest lack colors in the visible spectral range, such devices must use higher cost infrared detectors. Typically, the least expensive infrared imaging detectors relevant for such applications are uncooled detectors.
For some of the above applications, for example when the device must provide an alarm at cloud concentrations and size combinations above predetermined thresholds, quantitative data are required. To this end, these devices must use at least two spectral filters for filtering two selected wavelength bands, radiation in one wavelength band which is absorbed by the gas, and radiation in the other wavelength band which is not absorbed by the gas. The radiation in each wavelength band is imaged and analyzed separately. Device calibration methods and mathematical algorithms can be used to subsequently transform this quantitative data into scenes where the cloud image (when present) is shown, and where the quantitative information on the optical density of the specific gas of interest is stored pixel by pixel. Such a double filtering configuration is necessary in order to take into account contributions to the signals due to background infrared self-emission and drifts thereof brought about by background temperature drifts. This double filtering can be achieved with a spectral scanning method in which there is movement of an optical component of the device, such as, for example, an interferometer, a set of band pass filters mounted on a rotating wheel, or a scanning mirror to gather spectral information. Devices based on uncooled detectors must be designed with a large focusing lens numerical aperture (low f-number) in order to increase detector sensitivity to the radiation of interest relative to environment radiation. This is due to the fact that such detectors have a wide field of view. Designing an optical system with such a low f-number can be achieved with the above mentioned moving components. However, movements of optical components causes decreased system reliability, thereby increasing maintenance and operating cost of the device. In order to reduce maintenance and cost, filtering techniques can be implemented without moving parts via prisms, beam splitters, or beam combiners. However, such techniques have the effect of decreasing the focusing lens numerical aperture, thereby decreasing the sensitivity of the system to the radiation of the scene of interest relative to the environment radiation.